1. Field of the Invention
This invention relates to optical imaging systems in electronic reprographic equipment. More specifically, the invention relates to a system for correction of optical defocus and related optical system errors by correction of electrical signals representative of the optically produced image data.
2. Description of Related Developments
Electronic reprographic equipment and facsimile transmission equipment employ scanners for optically scanning an image, such as a document, and converting the optical information to an electrical image signal. Several types of optical systems are commonly employed to achieve the raster scanning of input documents and produce the representative electrical signal. FIG. 1 illustrates one of several configurations frequently used with reduction optical systems to scan a document and produce a reduced size image on a linear array of photosensors. This configuration is relatively tolerant of errors in the object conjugate of the lens; i.e., errors in the optical path length from the lens, through the mirrors, to the original document. Variations in this path length of .+-.1 millimeters, while causing noticeable change in magnification, will typically have little degrading effect on the focus, or sharpness, of the optical image formed on the photosensor array. Since only one compound lens is used, however, there may exist a variation in image quality along the linear portion of the image sensed by the photosensor array. Variations in image quality from the end of such a scanned image line to the center of this line frequently occur with a reasonable degree of symmetry about the center of the scanned image. These variations in quality may, for example, be caused by the curvature of the surface of best focus of the lens, or by other related lens aberrations which are well known to vary with the distance from the center of the optical field. The mathematical description of these aberrations can be approximately determined from lens design data, but further aberrations occur due to small errors which result from the lens fabrication process. Other single lens optical systems, such as those frequently referred to as "full-rate/half-rate" scanners, have similar optical characteristics. As a result, scanners using single reduction lens optics may illustrate a decrease in image quality at the edges of the scan line, even when the center of scan is in good focus.
A second class of optical systems frequently used for document scanning in electronic reprographic equipment is the "full-width" scanner type, shown in FIG. 2. Here, an array of lenses extends (into the page in the figure) the full extent of the line on the input document which is to be imaged at unity magnification onto the full-width photosensor array. Full-width scanners have been developed which utilize amorphous or crystalline silicon photosensor arrays that offer the advantages of high responsivity (which yields high scanning speeds), low illumination requirements (which reduces power consumption) and compactness. These scanners require compact, full width lens arrays to achieve these performance advantages. The most commonly used lenses for this purpose are gradient index fiber lens arrays, as illustrated in FIG. 3. While commercially available gradient index lens arrays provide good optical efficiency and excellent control of the unity magnification requirement, they have poor depth-of-field capabilities when compared with, for example, reduction optics designs; i.e., they are considerably more sensitive to errors in the object conjugate length than reduction optics designs. Typically, the depth-of-field for high efficiency gradient index lens arrays is approximately .+-.0.25 to .+-.0.50 millimeters.
FIG. 4 illustrates the depth-of-field characteristics of a typical gradient index lens array. The graph of FIG. 4 plots the modulation transfer function (MTF) achievable by the lens array as a function of defocus distance at a predetermined spatial frequency; for example, FIG. 4 shows these characteristics for a commercially available lens at 6 cycles per mm. The MTF value correlates directly to the fidelity level of the image from the lens array. By selecting a desired level of MTF (and thus image fidelity) the curves show the defocus distances (or depth-of-field) of the lens array necessary to maintain the desired MTF. In FIG. 4, the zero on the abscissa represents the best focus position and the small divisions along the abscissa are tenths of a millimeter. Curve M represents the characteristics of the lens in the main scanning direction along a line coincident with the line of photosensors, and curve S represents the characteristics of the lens in the subscanning direction; i.e., perpendicular to the main scanning direction. From FIG. 4, it is evident that at high levels of image fidelity (MTF), small variations in defocus distance can cause unacceptable blurring and that the amount of blur varies with the scan direction (i.e., the lens becomes increasingly anamorphic in this loss of quality as the focus error increases). As a result, optics/sensor architectures employing such full-width lens arrays frequently do not provide sufficient image quality or resolution to meet image fidelity requirements, thereby limiting the use of such scanner designs.